1. The Field of the Invention
The present invention relates to a method and apparatus for improving the placement of cement plugs.
2. The Prior Art
Cement plugs in wellbores often fail because the fluid in the well mixes with the cement slurry as the slurry is placed. When the slurry sets up, it does not form a solid competent plug. The current techniques rely on the cement slurry to sweep other fluids out of the way by simple fluid displacement. However, the cement slurry tends to finger through any gelled fluid in the wellbore rather than to spread out and displace gelled fluid from the full bore of the well. Various mechanical means have been used to stir and fluidize the fluid in the wellbore to improve displacement, but a cement slurry is not positively confined in such a way that it can be prevented from mixing with other fluid in the well. Consequently, large volumes of cement slurry are used to improve the likelihood of obtaining adequate displacement of the other fluids. This strategy has had limited success because of the stability of flow paths through gelled fluid, once such paths are established.